This invention relates to a gas flow measuring apparatus, and particularly to an apparatus for measuring the amount of flow of air which an engine, for example, sucks.
There has been proposed an air flow measuring apparatus of the type in which a suction duct of engine is provided with an air flow measuring tube including therein an electric heater and temperature dependent resistors each disposed in front of and in the rear of the heater, the output signals therefrom being used for measuring the flow of sucked air (gas to be measured).
This apparatus has advantages of small size and simple construction and of measuring accuracy being independent of the conditions in which apparatus is mounted. However, the construction is such that the two temperature dependent resistors and two additional reference resistors are used to constitute a bridge circuit to which a DC voltage is applied and that a diagonal voltage of the bridge is amplified by a DC differential amplifier and then used to control a voltage (or current) applied to the electric heater, thereby measuring air flow. Thus, current flows in the bridge at all times and hence a drift occurs degrading the measurement accuracy. In addition, a temperature drift takes place in the DC differential amplifier, also deteriorating the measurement accuracy.
Use of special low-drift elements in circuits to cope with the above problem will improve the measurement accuracy to some extent, but increase cost as compared with use of general-purpose elements. In addition, this can not fundamentally improve the accuracy.